Prior work in our laboratory has indicated that BALB/c mice bearing the MOPC-21 myeloma or the MCDV-12 leukemia contain a factor in their sera or peritoneal fluid which can be detected by virtue of its unique ability to selectively inactivate a particular DNA polymerase (designated R-1 polymerase) which is extractable from the MOPC-21 myeloma tumor and can be readily distinguished from the alpha, beta, gamma or viral DNA polymerases. The serum inhibitor appears to be a small DNA molecule which can also be detected by virtue of its ability to bind ethidium bromide and form a fluorescent complex. The inhibitor is not present in normal sera. Two principal objectives of this proposal are to determine whether the appearance of the inhibitor in body fluids or its presence in myeloma and leukemia cells is a property of the malignant state and to determine whether inhibitor is associated only with leukemia and myeloma cells in the mouse. Additionally, we wish to determine the level of the inhibitor (in serum and in spleens) as a function of age in mice from a high leukemia-incidence strain (AKR).